The Japanese patent application No. 55-110619, filed on Aug. 12, 1980, already discloses a device for generating fluid drops comprising an inlet connected to a source for feeding fluid thereto, a fluid chamber, a fluid outlet opening of said chamber and an actuation member located at least partially in said chamber, wherein said actuation member has a front surface area located in the vicinity of said outlet opening and facing it, and wherein said actuation member is responsive to an electric actuation signal fed thereto for causing a fluid drop to be generated at the outlet opening. The prior art device for generating fluid drops is connected to a source supplying said fluid under high pressure to the inlet opening. The actuation member consists of a vibrator located outside the fluid chamber of the device, which vibrator is secured to a horn extending into the interior of said fluid chamber, said horn having a front surface area tightly sealing against the inner wall of said fluid chamber in the area of said outlet opening, to thereby form a closing valve preventing the pressurised fluid in the chamber from flowing through said outlet opening as long as the vibrator is in its non-actuated condition. When feeding an actuation signal to the vibrator, it retracts said horn from said opening to thereby allow the fluid to be ejected from said outlet opening. The size of the drops generated by said device not only depends on the pressure of the fluid as generated by the source for feeding fluid to the inlet of the device, but also depends on numerous factors varying from device to device within a series of manufactured devices, since the dynamic behaviour of the valve member of said prior art device formed by said horn, the vibrator and defined by the relative location of the horn within said chamber varies from device to device. In order to avoid any irregularities in the generation of the drops, it thus turns out to be necessary to tune said prior art device so as to obtain similar drop generation properties of all devices originating from one manufacturing series.
The European patent application No. 82 307 017.2, publication No. 83877, filed on Dec. 31, 1982, discloses another kind of device for generating fluid drops which does not make use of a valve member closing and opening periodically the outlet opening of the device. This prior art device for generating fluid drops has its inlet continuously connected to a fluid chamber, which in turn is continuously connected to the outlet opening. A wall portion of the fluid chamber is made of a mechanically preloaded, deformed diaphragm which can be actuated by the transducer device. When feeding an actuation current to the transducer, said diaphragm is bent away from its preloaded position to thereby introduce a pressure wave in the fluid causing an ejection of a fluid droplet from the outlet opening. The size of the drop formed by actuating said transducer depends on numerous factors, like the degree of preloading said diaphragm, variations in the geometric dimensions of the transducer device, varying interengagement conditions between the transducer and diaphragm, and so on. Thus it is necessary to tune this prior art device to achieve uniform drop generating properties of all devices manufactured by making use of series production.
The prior, non-prepublished international application PCT/EP85/00724 and the corresponding prior, non-prepublished European application No. 85 116 306.3 (applicant's own) concerns a further device for generating fluid drops making use of a movable actuation member cooperating with an elastic, diaphragm-like partition wall separating the inlet from the outlet of said device and forming a kind of valve seat.
In view of this state of art, the present invention is based on the object to provide a device for generating fluid drops which has stable and reliably pre-determinable drop generation properties which do not vary between the different devices orginating from one manufacturing series, so that it is no longer necessary to individually tune each single drop generating device.
This object is achieved by a device for generating fluid drops in accordance with the invention.
The claimed device avoids any dependency of the size of the fluid drops on the properties of the source for feeding fluid to the inlet due to the fact that it works with a fluid supply under atmospheric pressure. The actuation member of the device in accordance with the present invention does not act as a valve for closing and opening the outlet opening, but has rather to be considered as a transmitter for ultrasonic waves which cause fluid drops to be ejected from said outlet opening. The actuation member is an integral member consisting of magneto-strictive material and is not a compound element having numerous elements which were required for forming the actuation members of prior art devices and which cause great variations in the actuation properties. The actuation member of the device in accordance with the present invention is integrally formed of magneto-strictive material. It has turned out that this very simple mechanical structure has a dynamic behaviour which can be reliably predetermined and which does not change from one device to the next within one manufacturing series. These constant and predictable properties are first of all achieved by the fact that the actuation member of the device in accordance with the present invention does not interfere with other devices, like diaphragms or means for preloading the actuation member. Moreover, the pressure wave generated by the actuation member is essentially independent of the exact distance between the front surface area of the rod-like integral member and the inner wall of the chamber surrounding the outlet opening which further enhances the accuracy in generating fluid drops.
Hence, the fluid drop generating device in accordance with the present invention replaces complicated mechanical structures which have been used and suggested until now by a simple device which does not require any tuning for achieving the desired drop generation properties.
Moreover, the device in accordance with the present invention meets high requirements for long-term stability and works with actuation signals at common drive voltages in the range of 5 to 25 volts.